The invention relates to envelope-type packages of the kind that became widely known from "smoked almond" inflight snacks on the airlines in the sixties, formed from foil, paper and plastic. A continuous sheet of this laminate with at least one surface being a thermoplastic layer is folded over onto itself about the longitudinal centerline with the thermoplastic inside, and the overlaid side edges are welded together to form a seam. The endless tube thus formed is then heat-sealed laterally to define the bottom of an individual package, which is filled with nuts, then sealed across the top, and finally cut from the continuous band as an individual bag.
Multiple-compartment packages are produced the same way but with an added longitudinal seam parallel to the side edge seams creating a welded bridge between adjacent packets. Since the purpose of multiple compartments is to permit mixing the ingredients without opening the bag, the bridge must have a frangible or separable internal barrier.
Multiple compartment mixing packages of this type are used with products containing volatile or chemically active elements which must be separated from one another until just prior to use, such as certain adhesives, cosmetics, and even foods. In addition to all the requirements demanded of single-produce packages, these multiple compartment packages must address the problem of producing a reliable yet frangible connection that will not permit seepage from one packet to the next, and will not separate during rough handling, yet will repeatedly and reliably fail before the perimeter seals give way when the packets are squeezed.
Early approaches to the bridge problem are typified by the instant inventor's earlier patents, and various other techniques initiating the use of dissimilar resins such as low density polyethylene and ionomer resin, with carefully controlled welding conditions, in an attempt to produce a peelable weld. Also, printed blocks of peelable coatings or fibrous material such as paper have been used as welding inhibitors in the dividing weld of a multiple compartment package. Such peelable coatings have not typically produced reliable and rugged enough frangible seals to survive the rigors of commercial distribution
A better approach has been the full-strength weld across an inter-laminate weakened area. See U.S. Pat. No. 3,608,709, issued Sep. 28, 1971. Typically such packages have to be made utilizing printing presses which can print a block of resist on one side of the package web in register with the graphics on the opposite side. Such two-side register printing presses are available but not common. In the known art, when a package web is thus printed in register on two sides, a secondary step is required wherein the entire inner surface of the web is extrusion-coated with a thermoplastic layer, trapping the resist block between the two layers of plastic. Because the resist prevents welding between two adjacent layers of thermoplastic, and because the overall extruded plastic coating welds in all other areas to the inner web surface, this process results in the manufacture of a package web with an inter-laminate weakened area at the resist block. Subsequently, the inter-laminate weakened area would only have the strength of the final extrusion coating rather than the combined strength of all of the layers of the composite, as detailed in the above-referenced patent. This process, while effective, required highly specialized manufacturing equipment and is only cost effective if done in very high quantities.
Another known method is to pre-print a resist block on a narrow thin web of thermoplastic and introduce this into the packaging machine. When such blocks are first printed on a narrow thermoplastic strip as is known in the art, and that strip is inserted into a form-fill-seal machine, it requires that the thermoplastic strip be continuously adjusted mechanically to longitudinally register the constant-feed webs forming the package walls with the merging resist strip, to ensure that the resist blocks at no point extend beyond the perimeter seal of the package. If the resist block is too long, it projects through the perimeter seal and creates a leakage path for the contents when the package is used. If too short, it produces a rectangular seal with sharp right angels when the frangible seal is broken. These right angle projections are points of weakness that often cause rupture of the package material and leakage of its contents. The proper dimension for the resist block is to extend up to, but not enter, the transverse heat seal region of the package. As these seals are typically on the order of 0.25 inch wide, this requires very close registration of the thermoplastic web which is often beyond the ability of commercial packaging equipment.
Since a resist block prevents welding, as opposed to the technique of incorporating an interlaminate peelable layer, there is an additional requirement that the resist block strip itself be welded around its perimeter to one package wall, in register with the package graphics on the opposite side of that package wall. Few packaging machines are capable of accomplishing these tasks. Thus one prior solution has been to process the package web offline with yet another machine to properly and precisely weld the thermoplastic frangible web to the package web. This additional process step and machinery adds substantial cost to the package.
The commonly used trade term, "shelf stability", describes the period of time it takes a product to deteriorate to commercial unacceptability. With some products, if the only barrier between separated elements is a thin thermoplastic layer, migration across the barrier is relatively rapid and predictable and results in low shelf stability. It is known in the art as represented in the inventor's U.S. Pat. No. 4,402,4402, issued Oct. 14, 1981, that substantial improvement in shelf stability can be obtained without changing the packaging materials, by separating active ingredients with multiple empty compartments rather than a single weld line. Spaced welds across the bridge area can be used to create miniature, empty versions of the product-containing packets, which lie as an obstacle course to migratory chemicals without substantially increasing the force necessary to rupture the barrier, since the compartments are ruptured sequentially as one of the packets is squeezed. This known technique, however, incurs a penalty in size and cost by forcing the addition of seal bars and associated mechanisms to the packaging machine. A considerably larger package is produced, with greater material cost, to accommodate the consecutive empty compartments.